Page 170 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

may coexist with or undergo transformation to brochantite because of its structural similarity
or that it may be a precursor to brochantite. Work by Selwyn and coworkers (i996), however,
has cast doubt on this supposition.
There are only two reports to date in which posnjakite has been identified from burial envi­
ronments: one by Schweizer (1991) on bronzes from a burial in a Swiss lake and the other by
Paradies and colleagues (i987) on buried copper pipes where the corrosion consisted almost
entirely of posnjakite under a thick, continuous biofilm containing many gram-negative organ­
isms. In both cases, microorganisms were an essential component in the corrosive events.
Posnjakite was identified in patina or as a patina constituent by Selwyn and colleagues
(1996) during a study of the corrosion and conservation of sixteen bronze monuments exposed
outdoors in Ottawa. One of them, Monument to Sir George Etienne Cartier by Louis-Philippe
Hébert (i850-i9i7), is shown in PLATE 35/ The surface shows typical vertical streaking due to
partial dissolution of the original patina that developed over the approximately seventy years
between the statue's unveiling and the time of this study. Posnjakite was found on statues that
had been exposed for both short and long periods, such as the statue of Queen Victoria unveiled
in 1901 (Selwyn et al. 1996).

Other basic sulfates New sulfates are being identified, and still others, no doubt,
await discovery, attesting to the complex nature of these com­
pounds as corrosion products. Bonatite, CuS0 4 -3H 2 0, has been reported only once as a patina
constituent—along with gypsum, antlerite, brochantite, and anglesite — from an exposed
bronze sculpture (Zachmann 1985). It may represent partially solubilized corrosion prod­
ucts mixed with stable mineral varieties. Strandberg and Johansson (1997 b) found yet another
basic sulfate, Cu 2 5 (OH) 3 S0 4 -2H 2 0, as a patina component on outdoor bronze sculptures,
although it is probably an intermediate product from the formation of brochantite and other
basic sulfates.
Guildite, CuFe(S0 4 ) 2 (OH)-4H 2 0, an uncommon Cu(II)Fe(III) basic sulfate, has been
reported in a corrosion context only on bronzes recovered from waterlogged environments
where sulfides are prevalent (Duncan and Ganiaris 1987); it may represent postexcavation oxi­
dation of a sulfide salt. The only other report of guildite is from its type location at the Union
Verde mine near the town of Jerome, Yavapai County, Arizona. The mineral was found there
F
after a fire, along with coquimbite, e 2 _ x Al x (S0 4 ) 3 -9H 2 0, and other sulfates.
A number of other unusual sulfate minerals have not yet been identified as corrosion prod­
ucts. They include krohnkite, Na 2 Cu(S0 4 ) 2 -2H 2 0; lautenthalite, PbCu 4 (S0 4 ) 2 (OH) 6 -3H 2 0;
6
and ktenasite, (Cu,Zn) 5 (S0 4 ) 2 (OH) 6 - H 2 0 . There is every reason to expect that ktenasite will
be found as a corrosion product on brass substrates because a closely related mineral, namuwite,

BASI C S U L F A T E S
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